Friction welded structural assembly and preform and method for same

ABSTRACT

Expanded structural assemblies and preforms and methods therefor are provided. Each preform can include at least two structural members in a stacked relationship, defining cells that can be inflated to expand the preform. Elongate members can be disposed between the structural members along the cells to define passages through which fluid can be received during expanding. Further, the structural members of the preform can be connected by friction stir weld joints, some of which can extend only partially through the preform so that the preform defines cells that can be expanded. More than one adjacent friction stir weld joint can be disposed between adjacent cells of the preform to define multiple-pass friction stir weld joints.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 10/781,419, filed on Feb. 18, 2004, which issued as U.S. Pat. No.7,048,175, which is a continuation-in-part of U.S. patent applicationSer. No. 10/742,325, filed on Dec. 19, 2003, now abandoned, each ofwhich is hereby incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to expanded structural assemblies and,more particularly, to an expanded structural assembly joined with aplurality of friction weld joints and to preforms and methods forforming such structural assemblies.

2) Description of Related Art

Expanded structural assemblies, and particularly honeycomb panels, areused in a variety of applications in the aerospace industry including,for example, flight control surfaces, acoustic suppression panels,aircraft flooring, other aircraft structural components, and the like.In addition, such expanded structural assemblies are also used in avariety of other industries and applications, including as structuralmembers for automobiles and other vehicles. Honeycomb panels and otherexpanded structural assemblies advantageously provide a combination ofhigh stiffness and low weight as compared to conventional panels formedfrom a solid material.

Conventional metal bonded aerospace honeycomb panels are produced bywelding or brazing thin “foil like” core sheets to thicker outboard facesheets in a flat configuration. The resulting flat aerospace honeycombscan be used as flat structural panels or further creep-stretch formedinto slightly curved shapes.

U.S. Pat. No. 6,537,682 to Colligan, titled “Application of FrictionStir Welding to Superplastically Formed Structural Assemblies,”describes a structural assembly formed by friction stir welding multiplestructural members and thereafter superplastically forming the membersto form the expanded assembly. The assembly can be formed by inflatingthe structural members in a die so that the assembly is formed to theshape of the die. The facing surfaces of the structural members can bepartially covered with oxide to prevent undesired thermo-compressingwelding from occurring adjacent the friction stir weld joints.

Although the methods of the prior art have proven successful in formingexpanded structural assemblies, there exists a continued need for animproved expanded structural assembly and methods and preforms forforming the same. Preferably, the method should be capable of formingexpanded structural assemblies of various shapes. Further, thestructural assemblies should provide consistent expanding of the cellsof the assemblies so that the assemblies are formed to the desiredshape.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method for forming preforms andexpanded structural assemblies therefrom. The preforms can include atleast two structural members in a stacked relationship with elongatemembers disposed therebetween and extending generally along a path of atleast one cell of the structural assembly and defining a passage throughwhich fluid can be received during expanding of the cells. Further, thestructural members of the preform can be connected by friction stir weldjoints, some of which can extend only partially through the preform sothat the preform defines cells that can be expanded.

According to one embodiment of the present invention, one or more coremembers can be positioned between first and second face members, and theface members can be friction welded to the core members with a frictionwelding tool that partially penetrates the core members. For example,the members can be friction stir welded with a rotating welding tool. Ifmultiple core members are provided, friction stir weld joints can beformed entirely within the core members, and elongate members can bedisposed between the core members to extend generally along the paths ofthe cells. A periphery of the preform can also be friction welded with atool that at least partially penetrates both of the face members. Theperiphery can be welded to define at least one fluid inlet in fluidconnection with the cells.

At least two adjacent friction weld joints can be formed betweenadjacent cells, and the adjacent friction weld joints can have acombined width that is greater than a thickness of each of thestructural members. The preform can be inflated by expanding the cells,e.g., to six-sided shapes that extend in a longitudinal direction sothat the cells of the structural assembly define a honeycombconfiguration. The preform can be expanded in a die cavity that definesa contour surface corresponding to a desired contour of the structuralassembly so that expanding the cells urges the structural membersoutward against the die cavity. The contour surface can define a complexcurve so that the structural assembly is formed to define the complexcurve of the contour surface. According to one aspect of the invention,the preform is heated to a superplastic forming temperature such thatthe preform is superplastically formed during said inflating step. Afterinflating the preform, the resulting structural assembly can be quenchedby circulating a coolant fluid therethrough. Alternatively, the preformcan be cold stretch formed to the desired shape. The elongate memberscan also be removed from the structural assembly.

The present invention also provides a friction welded preform and anexpanded structural assembly formed therefrom. The preform includes atleast two structural members in a stacked relationship. The structuralmembers can be formed of materials such as aluminum and aluminum alloys.A plurality of friction weld joints connect the structural members sothat the structural members define at least one cell between thefriction weld joints. A weld joint also extends at least partiallyaround a periphery of the structural members and defines a fluid inletfluidly connected to each cell so that the preform can be expanded by apressurized fluid that is injected through the fluid inlet and into thecells. At least one elongate member can be disposed between thestructural members of the preform so that the elongate member extendsgenerally along a path of each cell and maintains a passage between thestructural members.

The preform and structural assembly can include first and second facemembers with at least one core member therebetween, and the elongatemember can be disposed between the core members and each of the firstand second face members. A first of the friction weld joints can extendbetween the first face member and at least a portion of the core member,and a second friction weld joint can extend between the second facemember and at least a portion of the core member so that the firstfriction weld joint can be inflated away from the second face member andthe second friction weld joint can be inflated away from the first facemember. Further, the preform and structural assembly can include aplurality of core members, and some of the friction weld joints can bedisposed entirely between the core members such that the face membersare configured to be inflated away from the core members. According toone aspect of the invention, the preform and the structural assemblydefine at least two adjacent friction weld joints between adjacentcells. The adjacent friction weld joints can have a combined width thatis greater than a thickness of each of the structural members. At leastone side of each cell of the structural assembly can be defined by thefriction weld joints.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a perspective view illustrating an expanded structuralassembly formed according to one embodiment of the present invention;

FIG. 2 is a perspective view illustrating the structural members of apreform for forming the structural assembly of FIG. 1;

FIG. 3 is a section view illustrating the structural members of FIG. 2as seen along line 3-3 of FIG. 2;

FIG. 4 is a section view illustrating the preform for forming thestructural members of FIG. 1;

FIG. 4A is a perspective view illustrating a preform for forming astructural assembly according to another embodiment of the presentinvention, including weld joints having nonlinear weld connections;

FIG. 5 is a section view illustrating the preform of FIG. 4 configuredin a die cavity for expanding;

FIG. 6 is a perspective view of a structural assembly formed from thepreform of FIG. 4 according to another embodiment of the presentinvention;

FIG. 7 is a section view of a preform for forming a structural assemblyaccording to another embodiment of the present invention;

FIG. 8 is an elevation view of the structural assembly formed from thepreform of FIG. 7;

FIG. 9 is a section view of a preform for forming a structural assemblyaccording to yet another embodiment of the present invention;

FIG. 10 is an elevation view of the structural assembly formed from thepreform of FIG. 9; and

FIG. 11 is a section view of a preform for forming a structural assemblyaccording to still another embodiment of the present invention, thepreform having a braze material disposed between the structural members.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, this invention may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Referring now to the drawings and, in particular, FIG. 1, there isillustrated an expanded structural assembly 10 according to oneembodiment of the present invention. The structural assemblies 10 formedaccording to the present invention can be used in a variety ofapplications and structures, including aerospace applications such asflight control surfaces, acoustic suppression panels, aircraft flooringstructures, other aircraft structural components, and the like.Alternatively, the structural assemblies 10 can be used in a variety ofother industries and applications, including as structural members forautomobiles, watercraft, other vehicles, building structures, and thelike.

The structural assembly 10 illustrated in FIG. 1 includes threestructural members 12, 14, 16 and, in particular, a first face member12, a second face member 14, and a core member 16 disposed therebetweenso that first and second sides 18, 20 of the core member 16 are directedtoward the first and second face members 12, 14, respectively. Thestructural members 12, 14, 16 are connected by weld joints 22, 24, 26such as friction stir weld joints that extend partially or completelythrough the structural members 12, 14, 16. In particular, first weldjoints 22 connect the first face member 12 to the core member 16, andsecond weld joints 24 connect the second face member 14 to the coremember 16. A peripheral weld joint 26, connecting all of the structuralmembers 12, 14, 16, extends around a peripheral portion 28 of themembers 12, 14, 16. The structural assembly 10 defines cells 30 betweenthe members 12, 14, 16 and between the weld joints 22, 24, 26.

Each of the weld joints 22, 24, 26 can be formed by various weldingprocesses, such as friction stir welding. For example, FIGS. 2 and 3illustrate the structural members 12, 14, 16 during formation of apreform 40 (FIG. 4) that is subsequently used to form the structuralassembly 10. The first and second friction weld joints 22, 24 are formedby friction stir welding devices 50 a, 50 b having rotatable pins 52 a,52 b that are configured to partially penetrate the structural members12, 14, 16. For example, the first friction stir welding device 50 adefines a shoulder 54 a and the pin 52 a extending therefrom. Theshoulder 54 a and pin 52 a are rotated by a rotary actuator (not shown),and the pin 52 a is urged into the structural members 12, 16. The pin 52a is configured to extend through the first face member 12 and at leastpartially through the core member 16. The welding device 50 a is thenurged in a predetermined path along the structural members 12, 14, 16 toform the first friction stir weld joints 22, which join the first facemember 12 and the core member 16, without joining those members 12, 16to the second face member 14. Similarly, the second friction stirwelding device 50 b has a shoulder 54 b urged against the second facemember 14 so that the pin 52 b extends through the second face member14, thereby forming the second friction stir welding joints 24 thatextend at least partially through the core member 16 to join the secondface member 14 and the core member 16 without joining those members 14,16 to the first face member 12. A third friction stir welding device 50c has a pin 52 c that is structured to extend at least partially througheach of the structural members 12, 14, 16 so that the resulting joint 26at the periphery of the members 12, 14, 16 is a full-penetration jointwhich joins all of the members 12, 14, 16. While separate friction stirwelding devices 50 a, 50 b, 50 c are illustrated in FIGS. 2 and 3, it isappreciated that each of the weld joints 22, 24, 26 can be formed usinga single welding device, e.g., a welding device with an adjustable pinor shoulder to penetrate the members 12, 14, 16 to a selective depth asrequired. Friction welding is further described in U.S. Pat. No.5,460,317 to Thomas, et al.

Each friction stir welding device 50 a, 50 b, 50 c can be used to form aplurality of the friction stir weld joints 22, 24, 26, and each of theweld joints 22, 24, 26 can be a multiple-pass weld joint comprised ofmultiple single-pass weld connections 22 a, 24 a, 26 a that are disposedadjacent one another so that each of the weld joints 22, 24, 26 has acombined width that is greater than a width of each of the individualconnections 22 a, 24 a, 26 a. For example, as shown in FIGS. 2 and 3,the first and second welding devices 50 a, 50 b can be urged alongmultiple adjacent paths so that the resulting multiple-pass weld joints22, 24 have widths that are each approximately equal to the sum of thewidths of the adjacent weld connections 22 a, 24 a, respectively.Advantageously, the combined width of the weld joints 22, 24, 26 can bemade as wide as desired, regardless of constraints on the width of thepins 52 a, 52 b, 52 c and the width of each individual weld connection22 a, 24 a, 26 a. In some cases, the width of the weld joints 22, 24, 26can be wider than the thickness of the structural members 12, 14, 16that are being welded, and/or as wide as one side of the cells 30. Insome cases, the width of the weld joints 22, 24, 26 can be wider thanthe thickness of the structural members 12, 14, 16 that are beingwelded, and/or as wide as one side of the cells 30.

Each of the individual weld connections 22 a, 24 a, 26 a can besubstantially linear as shown in FIGS. 1 and 2. Alternatively, one ormore of the weld connections 22 a, 24 a, 26 a can define a nonlinearpath, such as a sinusoidal or otherwise curved path, or a zigzag,sawtooth, or otherwise nonlinear path. For example, as shown in FIG. 4A,each of the weld joints 22 includes two substantially linear weldconnections 22 a and a nonlinear weld connection 22 a′ defining asinusoidal curve therebetween. In other embodiments, the weld joints 22,24, 26 can include other configurations of linear and/or nonlinear weldconnections 22 a′. For example, each weld joint 22, 24, 26 can includeseveral substantially parallel nonlinear weld connections similar to thesinusoidal weld connections 22 a′ with or without linear weldconnections 22 a, 24 a, 26 a proximate thereto. Further, the linearand/or nonlinear weld connections 22 a, 22 a′ can be partiallyoverlapped. In either case, the use of nonlinear weld connections withor without the use of linear weld connections can increase the stiffnessof the resulting structural assembly 10

The first and second weld joints 22, 24 are disposed to define spacestherebetween so that the preform 40 defines the cells 30 that aresubsequently expanded to form the structural assembly 10. In addition,elongate members 51 such as cables or wires can be disposed between thestructural members 12, 14, 16 in the cells 30 before the structuralmembers 12, 14, 16 are welded. The elongate members 51 can extend in acontinuous longitudinal direction between the structural members 12, 14,16, and the elongate members 51 have a width smaller than the width ofthe cells 30 so that the elongate members 51 maintain a space or passage53 along the length of the cells 30 as shown in FIGS. 3 and 4. Theelongate members 51 are preferably formed of a material havingsufficient strength and heat resistance to maintain the passages 53despite the forces and heat associated with friction stir welding orotherwise joining of the structural members 12, 14, 16. For example, theelongate cables 51 can be formed of stainless steel wire, typically witha diameter of between about 0.05 inches and 0.25 inches. Generally,larger diameter wire can be used for thicker structural members 12, 14,16 or configurations requiring increased force therebetween duringwelding. Smaller diameter wire can be used if the weld joints 22, 24, 26are to be formed close together so that the structural members 12, 14,16 are not excessively deformed by the presence of the elongate members51 therebetween. Oxide films can also be provided on the surfaces of thestructural members 12, 14, 16 adjacent the weld joints 22, 24, 26, asdescribed in U.S. Pat. No. 6,537,682 to Colligan, the entirety of whichis incorporated herein by reference.

The preform 40, also referred to as a forming pack, can be expanded andformed to a desired configuration of the structural assembly 10. In thisregard, fluid connections 42 can be provided for supplying and,optionally, venting fluid from the cells 30 of the preform 40 and thestructurally assembly 10. As shown in FIG. 2, the face members 12, 14can be larger than the core member 16, e.g., so that the face members12, 14 extend beyond the core member 16 to form an overhang region 44.The first and second weld joints 22, 24 can be formed co-extensive withthe core member 16 so that the cells 30 end proximate to the overhangregion 44, and the cells 30 are therefore open to a space 46 definedbetween the face members 12, 14 in the overhang region 44. The thirdweld joint 26, which connects the face members 12, 14 and the coremember 16 can extend around the periphery 28 of the preform 40 to closethe space 46. The fluid connections 42 extend through the third weldjoint 26, or the fluid connections 42 can be connected to the preform 40at gaps provided in the third weld joint 26 so that the connections 42are fluidly connected to the space 46 within the preform 40. In otherembodiments, fluid connections can be provided on opposite sides of thepreform 40, such as in the case of a large preform 40 defining longcells 30, so that the fluid is provided to opposite sides of the preform40.

The preform 40 can be expanded by providing a pressurized fluid to thecells 30, thereby inflating the cells 30 to a desired configuration. Forexample, as shown in FIG. 5, the preform 40 can be positioned in a diecavity 60 defined by corresponding dies 62, 64, and the fluidconnections 42 can be connected to a pressurized fluid source 48. Thefluid source 48 can be configured to provide a gas, such as an inert gasat a pressure sufficient for inflating the cells 30. The gas can enterthe passages 53, which are maintained by the elongate members 51, andthe gas then expands the passages 53 to form the cells 30. The diecavity 60 can be defined by contoured surfaces of the dies 66, 68, whichcorrespond to a desired contour of the structural assembly 10. Forexample, the contoured surfaces 66, 68 of the dies 62, 64 can definecurves, angles, and the like. Further, the dies 62, 64 can definecomplex curvatures, i.e., contours having more than one axis ofcurvature, such that the structural members 10 formed in the die cavity60 define correspondingly complex curvatures. For example, the preform40 can be expanded in the die cavity 60 shown in FIG. 5 to form thestructural assembly 10 of FIG. 6, which includes face members 12, 14that are curved about multiple axes of rotation. As shown in FIG. 6, thestructural assembly 10 can be trimmed after forming, and the elongatemembers 51 can be removed from the cells 30 of the structural assembly10. With the opposite ends of the assembly 10 trimmed as shown in FIG.6, the cells 30 provide open passages that can be used, e.g., tocirculate liquids or gas. Alternatively, the assembly 10 can be leftuntrimmed after forming, and the elongate members 51 can be part of thefinished product. The elongate members 51 can be attached to the othermembers 12, 14, 16 of the structural assembly 10 by the peripheral weldjoint 26.

According to one embodiment of the present invention, the preform 40 issuperplastically formed to form the structural assembly 10. As is knownin the art, superplastic forming can be performed by heating the preform40 to a superplastic forming temperature and subjecting the preform 40to a pressure differential, i.e., in this case, a relatively highpressure within the cells 30 of the preform 40 and a low pressure in thedie cavity 60 outside the preform 40. Apparatuses and methods forsuperplastic forming are described in U.S. Pat. No. 5,420,400 to Matsen;U.S. Pat. No. 5,994,666 to Buldhaupt, et al.; U.S. Pat. No. 4,811,890 toDowling, et al.; and U.S. Pat. No. 4,181,000 to Hamilton, et al., eachof which is incorporated herein in its entirety by reference. In somecases, the pressure differential can be 400 psi or higher.

The preform 40 can be heated, e.g., to a temperature sufficient forsuperplastic forming, by an induction heater that induces a current inone or more susceptors 70 in thermal communication with the preform 40.The susceptors 70, which can be disposed in the dies 62, 64, can beconfigured to uniformly heat the preform 40 to a desired temperature.Such induction heating apparatuses and methods are described in U.S.Pat. No. 4,622,445 to Matsen and U.S. Pat. Nos. 6,566,635 and 6,180,932to Matsen, et al., each of which is incorporated herein in its entiretyby reference.

The preform 40 can alternatively be inflated or otherwise formed withouta high temperature superplastic forming process. Instead, the preform 40can be stretch formed at a relatively cool temperature, requiring noheating or heating only to a temperature below the superplastic formingtemperature, i.e., cold stretch forming. For example, preforms 40constructed of various aluminum alloys, such as Al 5083, can be stretchformed without first providing substantial heating to the preforms 40.Thus, preforms 40 formed of these aluminum alloys or otherstretch-formable materials can be inflated in the die cavity 60 asdescribed above without the use of the induction heater or other type ofheater. The stretch forming of the preform 40 can result in coldworking, thereby improving the strength or other materialcharacteristics of the preform 40. Gas at pressures up to about 1000psi, and typically between about 400 psi and 500 psi, can be used tocold stretch form the preform 40.

For those preforms 40 that are heated in the die cavity 60 duringforming, the resulting expanded structural assemblies 10 are preferablycooled before being removed from the die cavity 60. For example,preforms 40 that are heated and superplastically formed in the diecavity 60 to form the structural assembly 10 can be quenched in the diecavity 60. Quenching can be performed by circulating a quenching fluidthrough the die cavity 60. The quenching fluid can be a liquid or a gas,such as cool air. The quenching fluid can be circulated through theexpanded structural assembly 10, e.g., into one or more of the gasconnections 42, through the cells 30 of the assembly 10, and out of theassembly 10 through one or more of the gas connections 42. The gasconnections 42 can be provided on opposite ends of the structuralassembly 10 for quenching, so that the quenching fluid can be deliveredinto the assembly 10 at one end thereof and released from the assembly10 at the opposite end of the assembly 10. In addition, the die cavity60 can be opened, i.e., by lifting the first die 62 or lowering thesecond die 64, so that air or other fluid can contact the outside of thestructural assembly 10, thereby further cooling the assembly 10. Thequenching fluid can be provided at a temperature that is sufficientlycool for quenching the hot structural assembly 10 and cooling theassembly 10 to a temperature at which the assembly 10 can be removedfrom the die cavity 60. Further cooling can be performed after thestructural assembly 10 is removed from the die cavity 60, e.g., bycirculating a fluid through the cells 30 of the assembly 10 and/orcirculating a fluid outside the structural assembly 10.

Other material processing operations can also be performed on thestructural assembly 10, including additional heat treatment operations.For example, the assembly 10 can be aged by increasing the temperatureof the assembly 10 according to a predetermined temperature schedule,e.g., by heating the assembly 10 to an aging temperature of about 250°F. and maintaining the assembly 10 at that temperature for apredetermined duration. Aging and other heat treatment operations can beperformed while the assembly 10 is still in the die cavity 60 or afterthe assembly 10 has been removed therefrom.

For those preforms 40 that are cold stretched or otherwise formed at lowtemperatures, the resulting expanded structural assembly 10 can beremoved from the die cavity 60 immediately after forming. Alternatively,material processing operations such as an aging operation can beperformed on the assembly 10 while the assembly is still in the diecavity 60.

FIG. 7 illustrates a preform 40 that includes two core members 16 a, 16b disposed between the face members 12, 14. The cells 30 between theface members 12, 14 and core members 16 a, 16 b can be inflated orotherwise expanded to form the structural assembly 10, as shown in FIG.8. The elongate members 51 can also be disposed between the structuralmembers 12, 14, 16 a, 16 b as the members 12, 14, 16 a, 16 b are stackedto maintain the passages 53 along the length of the cells 30, and theelongate member 51 can be removed from the cells 30 after forming, asshown in FIG. 8. The preform 40 can be produced by first stacking thefirst and second core members 16 a, 16 b and forming friction stir weldjoints 25 that extend through the core members 25. The first and secondstructural members 12, 14 can then be disposed on the first and secondsides 18, 20 of the core members 16 a, 16 b. The first structural member12 can be welded to the first core member 16 a by forming partiallypenetrating friction stir weld joints 22 that extend through the firststructural member 12 and through at least a portion of the first coremember 16 a. The second structural member 14 can be welded to the secondcore member 16 b by forming partially penetrating friction stir weldjoints 24 that extend through the second structural member 14 andthrough at least a portion of the second core member 16 b.Alternatively, the first core member 16 a can first be welded to thefirst face members 12, and the remaining members 14, 16 b can then bewelded thereto, either individually or in combination. In any case, theweld joints 22, 24, 25 connect the adjacent structural members 12, 14,16 a, 16 b so that the cells 30 are defined between the weld joints 22,24, 25, and so that the cells 30 are defined between adjacent structuralmembers 12, 14, 16 a, 16 b, which can be deformed by inflation of thecells 30. Each of the weld joints 22, 24, 25 can be a multiple-passjoint that includes any number of parallel adjacent weld connections.The face members 12, 14 can extend beyond one or more edges of the coremembers 16 a, 16 b to define an overhang region 44 for defining thespace 46 that communicates with the cells 30, and the peripheral area 28of the members 12, 14, 16 a, 16 b can be joined with a weld joint (notshown) that extends therethrough to seal the space 46 and cells 30.

It is appreciated that any number of structural members 12, 14, 16 canbe welded to form a preform according to the present invention. Further,in some embodiments, the weld joints 22, 24, 25 can be disposed onlypartially through the preform 40, e.g., between one of the face members12, 14 and a portion of the core members 16 or between successivelystacked core members 16. For example, FIGS. 9 and 10 illustrate apreform 40 and an expanded structural assembly 10 formed therefrom. Thepreform 40 includes four core members 16 a, 16 b, 16 c, 16 d disposedbetween the face members 12, 14. Friction stir weld joints 22, 24, 25 a,25 b, 25 c connect the core members 16 a, 16 b, 16 c, 16 d and the facemembers 12, 14. During formation of the preform 40, the core members 16a, 16 b, 16 c, 16 d can be formed independently of the face members 12,14 and then connected to the face members 12, 14. Alternatively, thepreform 40 can be formed by stacking the structural members 12, 14, 16a, 16 b, 16 c, 16 d in a different operational order, e.g., bysuccessively stacking the core members 16 a, 16 b, 16 c, 16 d onto thefirst face member 12 and then connecting the second face member 14 tothe core members 16 a, 16 b, 16 c, 16 d. The resulting structuralassembly 10 can provide an increased stiffness relative to otherhoneycomb structural assemblies that are formed of fewer numbers ofmembers. In particular, the six-sheet configuration of FIG. 10 can beexpanded to a total height that is greater than the height of theassembly of FIG. 8. An increase in the total height of a structuralassembly generally increases the stiffness of the assembly, andtherefore an expanded assembly having six structural members as shown inFIG. 10 can generally be stiffer than an assembly having four structuralmembers as shown in FIG. 8. Thus, panels having multiple core members,such as the structural assembly 10 shown in FIG. 10, can be used in suchapplications where increased stiffness is required, such as in transportvehicles, building construction, and rocket fuselages. In some cases,the assembly 10 of FIG. 10 can provide a crash barrier for a vehicle orother structure that absorbs significant energy during deformation.

Advantageously, the friction welding of the joints 22, 24, 25 a, 25 b,25 c can refine the grain structure and improve the elongationproperties of the structural members 12, 14, 16 a, 16 b, 16 c, 16 d sothat the members 12, 14, 16 a, 16 b, 16 c, 16 d can be plasticallydeformed by cold stretch forming, superplastic forming, and the like.Further, as described above in connection with FIGS. 2 and 3, each ofthe friction weld joints 22, 24, 25 a, 25 b, 25 c can include multipleadjacent weld connections so that the adjacent weld connections define acombined multiple-pass weld joint that is wider than each of theindividual weld connections. As shown in FIGS. 7 and 8, the combinedwidth of the adjacent weld joints 22, 24, 25 a, 25 b, 25 c can be aboutas wide as one side of the cells 30. By providing multiple adjacent weldconnections, the bond between the structural members 12, 14, 16 a, 16 b,16 c, 16 d achieved by the weld joints 22, 24, 25 a, 25 b, 25 c can bemade stronger and the risk of deformation of the face members 12, 14 canbe reduced. For example, if the face members 12, 14 are provided in arelatively planar configuration, and stretch formed in the die cavity30, the core members 16 a, 16 b, 16 c, 16 d can exert a tensile forcebetween the face members 12, 14 while the preform 40 is being deformed.However, by forming wide weld joints, the tensile force associated withthe core members 16 a, 16 b, 16 c, 16 d can be distributed over a largerportion of the face members 12, 14, thereby reducing the likelihood ofundesirable local deformation of the face members 12, 14 occurring nearthe joints 22, 24, 25 a, 25 b, 25 c.

The elongate members 51 can be removed from the cells after forming, asshown in FIGS. 8 and 10. The expanded structural assembly 10 can also becut, machined, or otherwise trimmed to a desired shape and size. Variousfeatures can be provided on the structural assembly 10. For example,holes can be provided in the face members 12, 14 for receiving connectordevices such as rivets, bolts, screws, and the like. Additionally,plates or other members can be welded or otherwise connected to the facemembers 12, 14 at regions where high stresses are anticipated, such asnear holes in, or edges of, the face members 12, 14, where thestructural assembly 10 is to be connected to other members, and otherregions.

The structural members 12, 14, 16 of the present invention can be formedof a variety of materials, including various metals and metal alloys.Preferably, the preforms 40 and, hence, the structural assemblies 10 areformed of materials that can be friction welded to form the frictionweld joints 22, 24, 25 before superplastic or stretch forming of thepreforms 40. Materials that can be friction stir welded and formedaccording to the present invention include, but are not limited to,aluminum, aluminum alloys, nickel alloys, and stainless steel. Further,the structural members 12, 14, 16 can be formed of so called“unweldable” materials, i.e., materials that are characterized by a highthermal conductivity and that typically quickly dissipate heat away fromthe weld joints, materials that exhibit cracking along the weld joint asa result of stresses caused by thermal expansion, and/or materials thatinclude constituents that are typically outgassed during fusion welding.Unweldable materials produce relatively weak weld joints when weldedusing conventional fusion welding processes and, thus, are for the mostpart not used in the construction of superplastically formed assemblies.Such unweldable materials can include certain alloys of aluminum andaluminum-lithium. Advantageously, many of these materials possessspecial corrosion, fatigue, strength, or ductility characteristics thatare desired in certain applications. Further, the structural members 12,14, 16 can be formed of similar or dissimilar materials, which aretypically difficult or impossible to weld using conventional fusion orresistance welding processes.

In some embodiments, a braze material can also be provided between thestructural members before the members are welded and formed. The brazematerial can then be melted, e.g., during the forming process, so thatthe braze material substantially fills any space between the structuralmembers proximate to the weld joints 22, 24, 26. For example, as shownin FIG. 11, the braze material can be disposed as foil in longitudinalstrips 80 between the structural members 12, 14, 16 at the locations ofthe weld joints 22, 24, 26. Subsequently, the strips 80 of the brazematerial is melted. For example, the strips 80 of the braze material canbe melted in combination with the forming operation. In particular, thebraze material can have a melting temperature that is less than themaximum temperature of the forming operation. In one embodiment, theforming operation includes a temperature increase at or near the end ofthe forming operation, so that the strips 80 are melted after thestructural members 12, 14, 16 have been formed to the desired shape. Thebraze material preferably has a melting temperature that is lower thanthe melting temperature of the structural members 12, 14, 16. Forexample, the braze material can be an alloy including one or more ofaluminum, brass, copper, or zinc. Preferably, the braze materialsubstantially fills the space between the adjacent structural membersproximate to the weld joints 22, 24, 26. For example, the braze materialcan fill the spaces between the adjacent weld connections 22 a, 24 a, 26a. The braze material can increase the strength and stiffness of theresulting structural assembly 10 as well as increasing the assembly'sresistance to fatigue and corrosion.

Many modifications and other embodiments of the invention set forthherein will come to mind to one skilled in the art to which thisinvention pertains having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A method for manufacturing a friction welded, expanded structuralassembly, the method comprising: positioning at least two structuralmembers in a stacked relationship; disposing at least one elongatemember between the structural members, each elongate member extendingalong a path of a corresponding cell of the structural assembly anddefining a passage therealong; friction stir welding and thereby joiningthe structural members in a predetermined pattern to form a preformdefining at least one cell therebetween; and inflating the cell with apressurized fluid to expand the preform to a desired configuration ofthe structural assembly, wherein inflating the cell comprises inflatingat least that portion of the cell that surrounds and is proximate to therespective elongate member to a size larger than the respective elongatemember while the elongate member remains within the corresponding cellsuch that the elongate member maintains contact with only one of thestructural members with the contact being along a single line ofcontact, said friction stir welding step comprising forming a pluralityof multiple-pass friction stir weld joints having at least two adjacentfriction stir weld joints between adjacent cells of the preform suchthat the adjacent friction stir weld joints of each multiple-passfriction stir weld joint have a combined width greater than a width ofeach of the adjacent friction stir weld joints taken individually, saidstep of forming the plurality of multiple-pass friction stir weld jointscomprising forming each of a plurality of the adjacent friction stirweld joints of a multiple-pass friction stir weld joint so as to atleast partially penetrate at least two of the structural members suchthat the adjacent friction stir weld joints join at least two of thestructural members.
 2. A method according to claim 1 wherein saidpositioning step comprises positioning at least one core member betweenfirst and second face members such that the first and second facemembers are directed toward opposite surfaces of the core member, andwherein said friction stir welding step comprises friction stir weldingthe first face member to the core member with a friction stir weldingtool that penetrates the first face member and at least a portion of thecore member such that the welding tool does not penetrate the secondface member and friction stir welding the second face member to the coremember with a friction stir welding tool that penetrates the second facemember and a portion of the core member such that the tool does notpenetrate the first face member.
 3. A method according to claim 2wherein said friction stir welding step comprises friction stir weldinga plurality of the core members such that the core members are joined byfriction stir weld joints disposed entirely within the core members, andwherein said inflating step comprises inflating the first and secondface members positioned on opposite sides of the friction stir weldjoints disposed entirely within the core members such that the first andsecond face members are inflated away from the core members.
 4. A methodaccording to claim 2 wherein said inflating step comprises expanding aplurality of cells, at least some of the cells of the preform beinginflated to six-sided shapes extending in a longitudinal direction suchthat the cells of the structural assembly define a honeycombconfiguration.
 5. A method according to claim 2 further comprisingwelding a periphery of the preform to define at least one fluid inlet influid connection with the cells.
 6. A method according to claim 5wherein said second welding step comprises friction stir welding theperiphery of the preform with a rotating friction stir welding tool thatat least partially penetrates each of the first and second face members.7. A method according to claim 1 wherein said friction stir welding stepcomprises forming the adjacent friction stir weld joints with a combinedwidth greater than a thickness of each of the structural members.
 8. Amethod according to claim 1 wherein said inflating step comprisespositioning the preform in a die cavity defining a contour surfacecorresponding to a desired contour of the structural assembly andexpanding the cells to urge the structural members outward against thedie cavity.
 9. A method according to claim 8 further comprisingproviding at least one die defining the die cavity, the contour surfacedefining a complex curve such that said inflating step comprises formingthe structural assembly to define the complex curve of the contoursurface.
 10. A method according to claim 1 further comprising heatingthe preform to a superplastic forming temperature such that the preformis superplastically formed during said inflating step.
 11. A methodaccording to claim 8 further comprising circulating a coolant fluidthrough the structural assembly after said inflating step while thestructural assembly remains within the die cavity, thereby quenching thestructural assembly.
 12. A method according to claim 1 wherein saidinflating step comprises cold stretch forming the preform.
 13. A methodaccording to claim 1 further comprising providing the structuralmembers, at least one of the structural members comprising aluminum. 14.A method according to claim 1 further comprising providing a brazematerial between the structural members and melting the braze materialto substantially seal the weld joints formed by said friction stirwelding step.
 15. A method according to claim 1 wherein disposing atleast one elongate member comprises disposing at least one elongatemember having a width smaller than a width of the respective cell suchthat the elongate member maintains the passage between the structuralmembers generally along the path of the cell.
 16. A method according toclaim 1 further comprising removing the at least one elongate memberfrom the structural assembly following inflation of the cell.
 17. Amethod according to claim 1 wherein at least one of the friction stirweld joints of a multiple-pass friction weld joint has a nonlinearconfiguration and at least another of the friction stir weld joints hasa linear configuration, wherein the friction stir weld joints having thenonlinear configuration and the linear configuration define therebetweenportions of the structural members that remain unwelded.